Microbial fuel cell and method

ABSTRACT

A microbial fuel cell includes a cell housing having first and second chambers. The first chamber is adapted for containing a fluid including a biomas. The second chamber is adapted for containing an oxygenated fluid. A cathode extends into the cell housing second chamber and an anode segment of an electrode assembly extends into the cell housing first chamber. The electrode assembly has multiple, substantially aligned, fibers. The outer surfaces of the fibers of the anode segment are adapted for receiving a biofilm.

Cross-Reference to Related Applications

This application is a continuation of U.S. Pat. application Ser. No.12/164,186 filed Jun. 30, 2008 now U.S. Pat. No. 7,807,303.

BACKGROUND

This disclosure relates generally to fuel cells and methods of theirmanufacture and use. More particularly, the present disclosure relatesto fuel cells capable of operation by electrolyzing compounds in abiological system and methods of their manufacture and use.

There has long been interest in techniques for providing electricalpower from a power source that utilizes biological matter freelyavailable in the environment. One such area of interest is in developingmicrobial fuel cells (MFCs) as a means to treat wastewater moreefficiently by breaking down organic waste products and converting theenergy of their chemical bonds into electricity and hydrogen. Accordingto the May 2004 issue of Environmental Science & Technology, 46 trillionliters of household wastewater are treated annually in the United Statesat a cost of $25 billion. Importantly, the electricity required—mostlyfor aeration—constitutes 1.5% of the electricity used in the nation.Other nations have similar statistics.

Recently, researchers have shown the feasibility of using microbial fuelcells to generate electricity wherein the source of electricity is thechemical energy contained in the bonds of organic compounds which are aprinciple constituent of wastewater. Using laboratory scale microbialfuel cell reactors comprising a special anode, a simple cathode and asuitable proton exchange membrane (PEM) to separate the wet anode andcathode portions of the microbial fuel cell, energy densities in theorder of 30 watts/cubic meter have been generated.

The process uses bacteria, living in biofilms on the special anode, tobreak down the organics, separating electrons from protons. Theseelectrons and protons then travel to the cathode, the former via anexternal wire, the latter by diffusing through the electrolyte which isgenerally a substance that does not conduct electricity readily. In theelectricity-generating microbial fuel cells, the protons and electronscombine at the cathode with oxygen to form water. This consumption ofthe electrons allows more electrons to keep flowing from the anode tothe cathode as long as there is a source of chemical bonds (i.e. organicwaste) to fuel the reaction.

The first microbial fuel cells produced between 1 and 40 milliwatts ofpower per square meter (mW/m²) of anode electrode surface area. In thepast year researchers have been able to increase this more than 10 foldby demonstrating that they could generate power in the range of up to500 mW/m² using domestic wastewater and 1,500 mW/m² with a surrogate forwaste water comprising glucose and air. Demonstration of these latterpower densities has encouraged much discussion about the technicalrequirements to enable profitable commercial power production. In brief,improvements to the output power density by another factor of at least10 will be required in order to make the technology attractive on acommercial scale.

Today, scale-up for commercial uses has several challenges. For example,the current laboratory-scale prototypes use materials that aren't sturdyor robust enough to be used in a commercial system. Further,experimental microbial fuel cells are presently small in size and wouldneed to be much bigger (to compensate for the low power density),undoubtedly and unfortunately this would greatly increase the distancebetween anode and cathode which would slow diffusion of hydrogen fromthe former to the latter, further damping efficiency. To be competitive,the power density must more than double the maximum achieved so far i.e.8,500 mW/m².

SUMMARY

There is provided a microbial fuel cell comprising a cell housingdefining first and second chambers. The first chamber is adapted forcontaining a fluid including a biomas. The second chamber is adapted forcontaining an oxygenated fluid. A cathode extends into the cell housingsecond chamber and an anode segment of an electrode assembly extendsinto the cell housing first chamber. The electrode assembly hasmultiple, substantially aligned, fibers. The anode segment extends fromthe first end of the fibers to an intermediate location disposed betweenthe first and second ends of the fibers. The outer surfaces of thefibers of the anode segment are adapted for receiving a biofilm.

The electrode assembly also includes a bound segment extending from thesecond end of the fibers to the intermediate location. The outersurfaces of the fibers of the bound segment are impregnated or encasedwith a binder.

The microbial fuel cell further comprises a seal device disposedintermediate the bound segment and the cell housing. Together, the boundsegment, seal device and cell housing defining a fluid-tight boundary.

A protein exchange membrane disposed within the cell housing divides thecavity into the first and second chambers.

The microbial fuel cell further comprises a first chamber inlet adaptedfor receiving a first fluid stream containing a biomass and a firstchamber outlet adapted for discharging the first fluid stream. At leastone second chamber inlet is adapted for receiving a second fluid streamcontaining oxygen and at least one second chamber outlet is adapted fordischarging the second fluid stream. The fluid may be a gas or a liquidor a mixture thereof.

The electrode assembly may have a cylindrical-shape, with the boundsegment forming the cell housing and the anode segment extendingradially inward from the bound segment. A cylindrical-shaped proteinexchange membrane is disposed intermediate the cathode and the anodesegment, the membrane dividing the cavity into the first and secondchambers.

The microbial fuel further comprises at least one first chamber inletadapted for receiving a first fluid stream containing a biomass and atleast one first chamber outlet adapted for discharging the first fluidstream. The first chamber inlet and outlet each define an angle with theelectrode assembly whereby the first fluid stream generates a circularflow of the fluid in the first chamber. At least one second chamberinlet is adapted for receiving a second fluid stream containing oxygenand at least one second chamber outlet is adapted for discharging thesecond fluid stream.

The fuel cell may include a plurality of first chamber inlets andoutlets located at intervals along the electrode assembly.

The diameter of the first chamber inlet port may be greater than thediameter of the first chamber outlet port.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood and its numerous objectsand advantages will become apparent to those skilled in the art byreference to the accompanying drawings in which:

FIG. 1 is schematic diagram of a first embodiment of a microbial fuelcell in accordance with the disclosure;

FIG. 2 is an enlarged perspective view of the electrode assembly of themicrobial fuel cell of FIG. 1;

FIG. 3 is a simplified cross-sectional view of a second embodiment of amicrobial fuel cell in accordance with the disclosure;

FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 3;

FIG. 5 is a simplified cross-sectional view of a variation of themicrobial fuel cell of FIG. 3; and

FIG. 6 is an enlarged view of area VI of FIG. 4.

DETAILED DESCRIPTION

With reference to the drawings wherein like numerals represent likeparts throughout the several figures, a microbial fuel cell inaccordance with the present disclosure is generally designated by thenumeral 10, 10′. As used herein, the term “fiber” refers tonon-metallic, fibers which exhibit a desired level of electricalconductivity. The term “binder”, “binder resin”, or “resin” as usedherein refers to a matrix material that retains the fibers in place andmay provide for one or more mechanical or structural features. The term“biomas” as used herein refers to any organic matter from whichelectrons and protons may be separated when the organic matter issuspended or dissolved in a liquid. The term “biofilm” as used hereinrefers to any agent or catalyst that can separate electrons and protonsfrom a biomas.

FIG. 1 is schematic diagram of a first embodiment of a microbial fuelcell 10 in accordance with the disclosure. The cell housing 12 isdivided into first and second chambers 14, 16 by a protein exchangemembrane 17. The anode segment 18 of an electrode assembly 20 extendsdownwardly into the interior of the first chamber 14 and the cathode 22extends downwardly into the interior of the second chamber 16. The firstchamber inlet 24 is connected to a wastewater source (not shown) forreceiving a wastewater flow stream 26 containing waste biomass 28. Thefirst chamber outlet 30 discharges the treated wastewater 32 from thefirst chamber 14. The second chamber inlet 34 is connected to freshwatersource for receiving an oxygenated freshwater flow stream 36. The secondchamber outlet 38 discharges the oxygen depleted freshwater 40 from thesecond chamber 16. It should be appreciated that the dischargedfreshwater 40 may be oxygenated and recycled to the second chamber inlet34. Alternatively, an oxygen source (not shown) can be incorporated asan inlet 34 to the second chamber 16 and fed as a gas stream into thefluid contained in that chamber 16. The protein exchange membrane 17separating the first and second chambers 14, 16 prevents the relativelylarge oxygen molecules present in the second chamber 16 from diffusinginto the first chamber 14, while allowing passage of proteins andhydrogen molecules. It also keeps solids that may be present in thewastewater stream 26 within the first chamber 14 of the cell 10.Electrical conductors 42, 42′ extending from the electrode assembly 20and cathode 22 are connected to a load 44, completing an electricalcircuit as described below. The loads 44 powered by the microbial fuelcell may include the pumps providing the flow of wastewater andfreshwater, for example.

Referring to FIG. 2, the rod-shaped electrode assembly 20 comprisesmultiple aligned fibers 45 extending from a first end 46 to a second end48. A first, anode segment 18 extends from the first end 46 to anintermediate location 52 disposed between the first and second ends 46,48, and a second, bound segment 54 extends from the intermediatelocation 52 to the second end 48. In the bound segment 54, the fibers 45are impregnated or encased with a suitable binder resin 56. In the anodesegment 18, the fibers 45 are substantially unbound and extend freelyfrom the bound segment 54. As used herein, “unbound” fibers are fibersthat are not impregnated or encased by a binder, whereby the outersurface of each unbound fiber may be coated by a biofilm along theunbound length of fiber. The large surface areas of the free or unboundfibers 45 in the anode segment 18 become coated with a biofilm 58 duringoperation of the microbial fuel cell 10. A chemical reaction betweenbacteria in the biofilm 58 and biomas 28 in the wastewater stream 26reduces the biomas 28, and in so doing produces electricity.

Typically, there are many small diameter, large surface area fibers 45.they can number from 2 fibers to many millions, for example 100,000,000individual fibers 45. Each of the fibers 45 has a diameter that istypically in the range of about 0.1 to 100 microns. The free length ofthe portions of the fibers 45 in the anode segment 18 range from about 1cm to 100, or more meters.

Non-limiting examples of fibers 45 to be used are filled or unfilledtextile fibers such as polyester, rayon, polypropylene and nyloncomposite fibers containing appropriate conductive fillers such ascarbon black, carbon nanotubes, quaternary ammonium compounds, boronnitride, ionic salts and short lengths of conductive fibers. In manycases the fibers are required to have a high tensile and/or bendingstrength. One suitable fiber comprises a plurality of carbon fibers andis known as CarbonConX™ (Xerox Corp.). When combined with a binderresin, the carbon fibers can comprise carbon nano-filaments, each madefrom a single or multiwalled carbon nanotube (CNT) strand.

The multiple-segment electrode assembly 20 may be made by modificationof any suitable, commercial pultrusion, or molding, or compositingprocess, which combine continuous strands of carbon fiber 45 with abinder resin 56 to form a fiber-rich composite 60. Upon regularinterruption or shuttering of the resin flow to the fiber mass enteringfor example the pultrusion tooling where resin impregnation wouldgenerally occur, the fiber 45 does not combine with the resin 56 and alength of dry fiber progresses into, through, and out of the process.Thus, along the length of continuous fiber 45 would exist regularlyspaced resin impregnated segments separated by dry fiber segments thatcould be cut to form the electrode configuration shown in FIG. 2.

The binder 56 can be a polymer, ceramic, glass, or another suitablebinder. The binder 56 usually is a thermoplastic or thermoset polymerthat binds the fibers together with the necessary mechanical strength. Abinder 56 that does not affect the resistance properties of the fibers45 typically is selected. Suitable polymers include but are not limitedto acrylics, polyesters, polyamide, polyamide, polystyrene,polysulphone, and epoxies.

Since the bound segment 54 of the electrode assembly 20 is solid, it canserve as a liquid or fluid seal by integration with a seal device 57disposed between the bound segment and the wall of the cell 10. Aportion of the bound segment 54 may be coated with a suitable metal,such as gold, copper, nickel, tin, tin/lead and the like, by anysuitable coating means; including electrolysis plating, electrolyticplating, vacuum, gas, or other deposition, and the like, and, maycontain mechanical features such as a post, pin, spade, hole, and thelike to serve as an interconnection with conventional wiring 42 or othercircuit members of general use in a power circuit.

As discussed above, the anode segment 18 of the electrode assembly 20comprises a high surface area fibrous surface that in operation becomescoated with a biofilm 58. The biofilm 58 may comprise iron-reducingbacteria called cytochromes which are specialized enzymes known totransfer electrons to other proteins. In general however bacteria fromthe waste stream collects upon the anode segment forming a biofilm 58with conductive properties. The interaction of the biomas 28, thebacteria of the biofilm 58, and the anode segment 18 creates a source ofelectricity. In electricity-generating microbial fuel cells 10, 10′, theprotons 62 that flow through the electrolyte-membrane 17 and electrons64 flowing through the external circuit 42, 44, 42′ combine at thecathode 22 with oxygen carried by the oxygen-water stream 36 to formmore water molecules. This consumes the available electrons 64 allowingmore to keep flowing from the anode segment 18 to the cathode 22 throughthe circuit 42, 44, 42′ permitting energy to be harvested in thecircuit.

Research has shown that conventional microbial fuel cells suffer fromlow power density, with electron transfer from the biofilm to the anodebeing very slow. This low power density limited the amount ofelectricity that could be generated by the microbial fuel cell. Powerproduction could not be increased satisfactorily by merely scaling upthe size of the microbial fuel cell. Since power density is primarily afunction of the interface between the biofilm microbes and the anodesurface, it did not appear that this deficiency could be resolvedwithout modifying the characteristics of the biofilm. However, the freeor unbound fibers 45 in the anode segment 18 provide a significantincrease in available surface area, compared to conventional microbialfuel cell anodes, without scaling up the size of the microbial fuel cell10, 10′.

It should be appreciated that the thickness of the biofilm layer is animportant factor in determining the power density of a microbial fuelcell. If the biofilm is too thick the electrons have to travel too farto reach the anode segment, and if it is too thin there are too fewbacteria participating in the energy producing reactions. Low powerdensity results in both cases. In the embodiment 10′ shown in FIGS. 3-5,an optimum thickness for the biofilm 58 is maintained by directing theflow 98 of wastewater relative to the anode segment 68 such that arelatively small portion of the wastewater flow 98 enters the gaps 70(FIG. 6) between the separate fibers 72, thereby replacing thewastewater that has been depleted of biomas, and the major portion ofthe wastewater flow 98 moves past the surface 74 of the anode segment68, thereby creating a shear force that removes those portions of thebiofilm 58 that exceed the desired thickness.

The above-described flow pattern is achieved by utilizing arod-in-cylinder cell design where the electrode assembly 76 has acylindrical-shape, the cathode 22 is an axially extending rod positionedon the axis 78 of the electrode assembly 76, and a cylindrical-shapedprotein exchange membrane 80 disposed between the cathode 22 and theelectrode assembly 76 divides the microbial fuel cell into first andsecond chambers 82, 84. Similar to the first embodiment 10, the firstchamber 82 has at least one inlet 86 and at least one outlet 88 forreceiving and discharging, respectively, a wastewater flow stream 26containing waste biomass 28, and the second chamber 84 has an inlet 92for receiving an oxygenated freshwater flow stream 36 and an outlet 94for discharging an oxygen depleted freshwater flow 40.

To provide the effect of a large mass of wastewater moving across thesurface of the anode segment 68, the inlet 86 and outlets 88 for thewastewater stream 26, 32 are configured within the electrode assembly 76at an appropriate angle 108, 110 so as to impose a circular flow of thefluid stream 98 while it is contained within the microbial fuel cell10′. In the variation of FIGS. 3 and 4, multiple inlet ports 100 andoutlet ports 102 are located at intervals along the electrode assembly76 to enable a sufficient total fluid flow. In the variation of FIG. 5,the inlet port 100′ has a diameter that is larger than the diameter ofthe outlet port 102′, thereby assuring a pressure difference betweeninlet 86′ and outlet 88′ to enable a sufficient fluid flow.

As stated above, the electrode assembly 76 forms the outer wall of themicrobial fuel cell 10′. With additional reference to FIG. 6, theelectrode assembly has a cylindrical shape with an outer bound segment104 and an inner anode segment 68. Similar to the first embodiment 10,the fibers 72 are impregnated or encased with a suitable binder resin106 in the bound segment 104. In the anode segment 68, the fibers 72extend in a radial direction perpendicular or nearly perpendicular tothe bound segment 104. The surface areas of the free or unbound fibers72 in the anode segment 68 become coated with biofilm 58 duringoperation of the microbial fuel cell 10′.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A microbial fuel cell comprising: a cell housing defining first andsecond chambers, the first chamber being adapted for containing a fluidincluding a biomas, the second chamber being adapted for containing anoxygenated fluid; a cathode extending into the cell housing secondchamber; and an electrode assembly having a plurality of substantiallyaligned fibers, each of the fibers extending from a first end to asecond end and having an outer surface, the electrode assembly includingan anode segment and a bound segment, the anode segment extending fromthe first end of the fibers to an intermediate location disposed betweenthe first and second ends of the fibers the bound segment extending fromthe second end of the fibers to the intermediate location, the anodesegment extending into the cell housing first chamber, the outersurfaces of the fibers of the anode segment being adapted for receivinga biofilm, the outer surfaces of the fibers of the bound segment beingimpregnated with a binder.
 2. The microbial fuel cell of claim 1 whereinthe electrode assembly has between 100 and 100,000,000 fibers.
 3. Themicrobial fuel cell of claim 2 wherein the electrode assembly hasbetween 500 and 300,000 fibers.
 4. The microbial fuel cell of claim 1wherein the fibers have a diameter that is between 0.1 and 100 microns.5. The microbial fuel cell of claim 1 wherein the anode segment extendsfrom 1 cm to 100 meters from the bound segment.
 6. The microbial fuelcell of claim 1 further comprising a seal device disposed intermediatethe bound segment and the cell housing, the bound segment, seal deviceand cell housing defining a fluid-tight boundary.
 7. The microbial fuelcell of claim 1 further comprising a membrane disposed within the cellhousing, the membrane dividing the cavity into the first and secondchambers.
 8. The microbial fuel cell of claim 7 further comprising afirst chamber inlet adapted for receiving a first fluid streamcontaining a biomass and a first chamber outlet adapted for dischargingthe first fluid stream; and a second chamber inlet adapted for receivinga second fluid stream containing oxygen and a second chamber outletadapted for discharging the second fluid stream.
 9. The microbial fuelcell of claim 1 wherein the electrode assembly has a cylindrical-shape,the bound segment forming the cell housing and the anode segmentextending radially inward from the bound segment, the cathode beingdisposed within a cavity defined by the electrode assembly.
 10. Themicrobial fuel cell of claim 9 further comprising a cylindrical-shapedmembrane disposed within the cavity intermediate the cathode and theanode segment, the membrane dividing the cavity into the first andsecond chambers.
 11. The microbial fuel cell of claim 10 furthercomprising at least one first chamber inlet adapted for receiving afirst fluid stream containing a biomass and at least one first chamberoutlet adapted for discharging the first fluid stream, the first chamberinlet and outlet each defining an angle with the electrode assemblywhereby the first fluid stream generates a circular flow of the fluid inthe first chamber; and at least one second chamber inlet adapted forreceiving a second fluid stream containing oxygen and at least onesecond chamber outlet adapted for discharging the second fluid stream.12. The microbial fuel cell of claim 11 wherein the fuel cell comprisesa plurality of first chamber inlets and a plurality of first chamberoutlets located at intervals along the electrode assembly.
 13. Themicrobial fuel cell of claim 11 wherein the first chamber inlet andoutlet each have a diameter, the diameter of the first chamber inletport being greater than the diameter of the first chamber outlet port.14. The microbial fuel cell of claim 9 wherein the fibers of the anodesegment extend in a radial direction substantially perpendicular to thebound segment.
 15. A microbial fuel cell comprising: a cell housing; amembrane disposed within the cell housing, the membrane dividing thehousing into first and second chambers, the first chamber being adaptedfor containing a fluid including a biomas, the second chamber beingadapted for containing an oxygenated fluid; a cathode extending into thecell housing second chamber; and an electrode assembly having aplurality of substantially aligned fibers, each of the fibers extendingfrom a first end to a second end and having an outer surface, theelectrode assembly including an anode segment and a bound segment, theanode segment extending from the first end of the fibers to anintermediate location disposed between the first and second ends of thefibers, the bound segment extending from the second end of the fibers tothe intermediate location, the outer surfaces of the fibers of the anodesegment being adapted for receiving a biofilm, the outer surfaces of thefibers of the bound segment being impregnated or encased with a binder.16. The microbial fuel cell of claim 15 wherein the electrode assemblyhas a cylindrical-shape defining an axis, the bound segment forming thecell housing and the anode segment extending radially inward from thebound segment, the cathode being substantially disposed on the axis ofthe electrode assembly and the membrane being disposed intermediate theanode segment and the cathode.
 17. The microbial fuel cell of claim 16further comprising: a least one first chamber inlet adapted forreceiving a first fluid stream containing a biomass and at least onefirst chamber outlet adapted for discharging the first fluid stream, thefirst chamber inlet and outlet each have a diameter, the diameter of thefirst chamber inlet port being greater than the diameter of the firstchamber outlet port, the first chamber inlet and outlet each defining anangle with the electrode assembly whereby the first fluid streamgenerates a circular flow of the fluid in the first chamber; and atleast one second chamber inlet adapted for receiving a second fluidstream containing oxygen and at least one second chamber outlet adaptedfor discharging the second fluid stream.
 18. The microbial fuel cell ofclaim 16 further comprising: a plurality of first chamber inlets adaptedfor receiving a first fluid stream containing a biomass and a pluralityof first chamber outlets adapted for discharging the first fluid stream,the first chamber inlets and outlets being located at intervals alongthe electrode assembly, the first chamber inlets and outlets eachdefining an angle with the electrode assembly whereby the first fluidstream generates a circular flow of the fluid in the first chamber; anda second chamber inlet adapted for receiving a second fluid streamcontaining oxygen and a second chamber inlet adapted for discharging thesecond fluid stream.
 19. The microbial fuel cell of claim 15 wherein thefibers of the anode segment extend in a radial direction substantiallyperpendicular to the bound segment.